In the second year of this project we described regional changes in brain oxytocin receptors associated with the onset of maternal behavior. In this, the third year of this project, we further investigated the role of oxytocin in maternal behavior by studying the regulation of oxytocin gene expression using in situ hybridization. Preliminary results reveal increased oxytocin mRNA in the rostal paraventricular nucleus of the hypothalamus following parturition. Electrolytic lesions of this region altered the initiation but not the maintenance of maternal behavior. The role of gonadal steroids on oxytocin receptor number and oxytocin gene expression is currently under study. We have obtained an oxytocin analogue which is highly selective for the oxytocin receptor and which can be labeled with 125I for use with in vitro receptor autoradiography. This long-awaited compound, Peptide VI, is the necessary tool to answer a number of questions about the physiologic regulation of oxytocin receptors.